Block copolymers are well known and have been used commercially as components in adhesives, as melt processable rubbers, impact resistant thermoplastics, and as compatibilizers, or "surfactants", for emulsifying polymer-polymer blends.
A block copolymer is created when two or more polymeric segments, or blocks, of different chemical composition are covalently bonded in an end-to-end fashion. Block copolymers have certain advantages over blends. Firstly, the segments are covalently bonded to each other, thereby eliminating the interface problem. Secondly, block copolymers can be used to strengthen blends of immiscible polymers by serving as "emulsifiers," which encourage physical connections between the phase, and thus improve the interfacial adhesion and load transferring capability of the components.
While a wide variety of block copolymer architectures are possible, most block copolymers of interest involve the covalent bonding of hard plastic segments which are crystalline or glassy, to elastomeric blocks forming thermoplastic elastomers. Other block copolymers, such as rubber-rubber, glass-glass, and glass-crystalline block copolymers are also possible and may have commercial importance. Two common types of block copolymer structures are the diblock and triblock forms. However, multiblock copolymers, in which more than three segments are bonded together, are also desirable.
Block copolymers are similar to, yet distinct from, tapered polymers. In a tapered copolymer the composition of comonomer is gradually varied from one end of the polymer to the other. Tapered copolymers are commercially used as viscosity modifiers, oil additives, thermoplastic elastomers, and impact-resistant plastics.
Block copolymers have been made by anionic polymerization routes. Butadiene-isoprene block copolymers have been synthesized using the sequential addition technique. In sequential addition, a certain amount of one of the monomers is contacted with the catalyst. Once the monomer has reacted to extinction, forming the first block, a certain amount of the second monomer species is introduced and allowed to react to form the second block. The process may be repeated as desired using the same or other anionically polymerizable monomers.
Likewise, olefin based block copolymers have not been successfully synthesized through coordination catalysis. Several difficulties arise in the use of known coordination catalysts for the block copolymerization of olefins. Among those are the fact that conventional catalysts are typically multi-sited, and a significant fraction of the active sites are unstable. This leads to random chain initiation and termination which, in turn, lowers the theoretical block copolymer yield. What is desired, and what practice of this invention provides, is a catalyst system with well-characterized structure and reactivity which has a single active site. The system should have well-defined and stable polymerization kinetics and be free of aluminum alkyls or other chain transfer agents.